1. Field of the Invention
The present invention relates to a chip resistor for surface-mounting on a printed circuit board and to a method of making the same.
2. Description of the Related Art
As is well known, various types of chip devices have been developed as components for constituting electric circuits. An example of such chip devices is a surface-mounting-type chip resistor (designated by a reference sign 21 as a whole) as shown in FIG. 15. The resistor 21 includes a rectangular substrate 22 formed of alumina ceramic material. As shown in FIG. 15, the substrate 22 has an upper surface 22a, side surfaces 22b and a lower surface 22c. The resistor 21 includes a pair of first upper electrodes 23 formed on the upper surface 22a, side electrodes 24 formed on the respective side surfaces 22b, and lower electrodes 25 formed on the lower surface 22c. The upper surface 22a of the substrate 22 is formed with a resistor element 26 connecting the first upper electrodes 23 to each other. The resistor element 26 is covered with a protective coating layer 27. Further, the protective coating layer 27 is covered with an overcoat layer 29. Each first upper electrode 23 has an upper surface formed with a second upper electrode 28.
The resistor 21 may be formed utilizing an aggregate board 11 as shown in FIG. 16, which is made of alumina ceramic material. The aggregate board 11 has a size capable of simultaneously providing a plurality of identical resistors. Specifically, the aggregate board 11 is sectioned into a plurality of rectangular regions 12. Each of the rectangular regions 12 corresponds to one resistor 21. In the figure, reference signs 13, 14 indicate excess portions of the aggregate board 11. The aggregate board 11 will be divided along the excess portions using a dicing cutter for example.
As shown in FIG. 16, the aggregate board 11 has an upper surface on which a plurality of conductor pieces 23xe2x80x2 are arranged in a matrix. Each of the conductor pieces 23xe2x80x2 extends across the corresponding cutting line 13. Each rectangular region 12 is overlapped by two conductor pieces 23xe2x80x2 which are spaced from each other along a cutting line 14. The overlapping regions finally become the upper electrodes 23 shown in FIG. 15.
After the conductor pieces 23xe2x80x2 are formed, necessary parts such as a resistor layer corresponding to the resistor element 26 (FIG. 15) and a protective coating layer and the like are formed. Then, at an appropriate stage, the aggregate board 11 together with the conductor pieces 23xe2x80x2 and the like formed thereon are divided along the cutting lines (excess portions) 13.
Thereafter, side conductor layers (corresponding to the side electrodes 24 of FIG. 15) are formed on the cutting surface of the substrate 11. By subsequently dividing the substrate 11 along the cutting lines (excess portions) 14, the plural rectangular regions 12 are completely separated from each other. Finally, plating is applied to the electrodes 24, 25 and 28 shown in FIG. 15, thereby providing the resistor 21 as a final product.
Although the above method is capable of making a plurality of resistors from one aggregate board and hence has a high manufacturing efficiency, it also has the following drawbacks.
As described above, in the manufacturing process, the aggregate board 11 (and the conductor pieces 23xe2x80x2 and the like) are divided along the cutting lines 13. At this time, the rotation of the dicing cutter may raise the conductor pieces 23xe2x80x2. In such a case, as shown in FIG. 17, the first upper electrode 23 of the resistor 21 includes a rising portion at the edge thereof. As a result, the second upper electrode 28 formed on the first upper electrode 23 also rises.
Such a rising portion formed in the resistor 21 causes various problems. For example, the side electrode 24 may not be suitably connected to the first upper electrode 23 or the second upper electrode 28. Further, in solder-plating the second upper electrode 28, solder cannot be suitably applied to the rising portion.
The present invention is conceived under the circumstances described above. It is, therefore, an object of the present invention is to provide a chip resistor which is free from the rising of an electrode on the supporting substrate.
According to a first aspect of the present invention, there is provided a method of making a chip resistor. This method includes the following steps. First, an aggregate board is prepared which includes a first region and a second region which are spaced from each other via an excess portion. Then, a conductor pattern is formed which extends to bridge the first region and the second region. Subsequently, a resistor element is formed in each of the first region and the second region for connection to the conductor pattern. Then, the aggregate board is cut at the excess portion. The conductor pattern includes a thinner-walled portion extending across the excess portion and a thicker-walled portion connected to the thinner-walled portion and spaced from the excess portion.
According to the above structure, the conductor pattern is cut together with the substrate. However, since the conductor pattern is cut at the thinner-walled portion, the problem of rising as is in the prior art does not occur. Preferably, the thinner-walled portion has a thickness of 0.1-3.0 xcexcm, whereas the thicker-walled portion has a thickness of 5-25 xcexcm.
Preferably, the conductor pattern forming step includes a sub-step of applying a conductor paste for the thicker-walled portion and a sub-step of applying a conductor paste for the thinner-walled portion.
Preferably, the conductor paste for the thicker-walled portion and the conductor paste for the thinner-walled portion are baked simultaneously.
Preferably, the conductor paste for the thicker-walled portion and the conductor paste for the thinner-walled portion are made of a same material.
Preferably, the method according to the present invention further comprises the step of forming a resistance adjusting groove in the resistor element.
According to a second aspect of the present invention, there is provided a chip resistor comprising an insulating substrate having an upper surface and a side surface, a first conductor pattern formed on the upper surface, a resistor element connected to the first conductor pattern. The first conductor pattern includes a thinner-walled portion contacting the upper surface, and a thicker-walled portion connected to the thinner-walled portion and contacting the upper surface. The thinner-walled portion is spaced from the resistor element and extends up to the side surface. The thicker-walled portion contacts the resistor element and is spaced from the side surface.
Preferably, the resistor further includes a second conductor pattern extending on the first conductor pattern. The second conductor pattern contacts both of the thinner-walled portion and the thicker-walled portion.
Preferably, the thinner-walled portion has a thickness of 0.1-3.0 xcexcm, whereas the thicker-walled portion has a thickness of 5-25 xcexcm.
Other features and advantages of the present invention will become clearer from the description of the preferred embodiment given below with reference to the accompanying drawings.